Ubiquitin and fusarium resistance: Lessons from wheat cDNAS conferring deoxynivalenol resistance in yeast

The production of the trichothecene mycotoxin deoxynivalenol (DON) is an important virulence factor of the plant pathogenic fungus Fusarium graminearum on wheat. We have engineered a DON sensitive yeast strain and constructed a cDNA library from DON treated wheat suspension culture cells in a yeast expression vector. The library was used to select DON resistance conferring clones. Besides ORFs of unknown function, we found 3 classes of cDNAs that in addition to DON resistance conferred hypersensitivity to hygromycin and canavanine. The predicted functions of several of the wheat cDNAs (putative E3 ligase, ubiquitin specific protease, proteasome subunit) suggested a role for ubiquitin-proteasome mediated protein degradation in DON resistance. Results with a coupled wheat germ in vitro translation system and a GUS-luciferase fusion gene showed that DON is a powerful translation elongation inhibitor. The truncated proteins formed in the presence of DON most likely lead to ubiquitin depletion and consequently growth inhibition in yeast. Ubiquitin is essential for many processes in plants, including plant defense. Our results warrant the re-evaluation of the relevance of proteasome system components found to be differentially regulated during Fusarium infection.